I can't believe that it hasn't been suggested before but my search for "metronome" indicates that it hasn't.

I have a metronome app but of course I have to get out of iGigBook, then setup the metronome, then get back into iGigBook. Awkward. And more over, if the product had a metronome feature it could remember and associate the beats per minute etc. with the song.

It would be really slick if the metronome had some basic drum machine capabilities! Or ... implement a plug in architecture so that I could buy a metronome/drum-machine (operating according to iGigBooks plug-in architecture) from anyone who chose to implement one.